The present invention relates to the field of direct sequence spread spectrum systems and deals more particularly with multiuser detection and interference suppression in direct sequence spread spectrum systems in which all users employ same spreading code.
Wireless communication has become one of the most indispensable technologies of modern world. In the beginning, time division and frequency division multiplexing techniques were used for communicating multiple signals using one channel. In these techniques, frequency of conventional wireless signals, like as in a conventional radio, remains constant. Hence, the bandwidth is limited in these techniques. Also, the transmitted data can very easily be retrieved by anyone having access to the bandwidth used. However, these techniques have certain limitations associated with them. Firstly, the signal is subject to interference from other signals having frequencies that are same or are close to the frequency of the signal. Interference can be deliberate, like jamming, or unintended, like side channel interference in conventional radio transmission. Interference from other signals affects the quality of communication and it becomes difficult to recover the signal at the receiver. Secondly, the signal is very easy to intercept, which is not ideal for situations where confidentiality of the information sent is of paramount importance.
Spread spectrum technology takes care of the shortcomings of conventional wireless communication technologies. Spread spectrum radio communication technology was developed for use by military because it resists jamming and uses wideband signals that are difficult to detect. It is only in the recent years that the utility of spread spectrum technology for commercial and industrial purposes has been realized.
Spread spectrum is a form of wireless communications in which the signal to be transmitted is spread in order to occupy a bandwidth much greater than what is necessary to send the information. This makes the signal immune to interference, difficult to intercept and also makes multiuser access possible. Frequency hopping, time hopping and direct sequence are three ways to spread the bandwidth of a signal. In frequency hopping, the signal is switched across frequencies in the given bandwidth. In time hopping, the signal is transmitted in short bursts at different instances of time. In direct sequence, the data to be transmitted is directly coded at a high frequency. The data is said to be composed of symbols. Bandwidth of the signal is spread by means of a code that is independent of the data to be transmitted. The information data modulates the code. This code is called spreading code and it is a pseudo-random code. The resulting signal modulates a carrier signal. The signal thus obtained is then amplified and transmitted. The spreading code can be short or long. A short spreading code is the one that has period equal to the symbol period of the information signal. A long spreading code is the one that is aperiodic or has period much greater than the symbol period. The same code is reconstructed at the receiver end so that synchronous detection is possible. The receiver synchronizes with the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.
Code division multiple access (CDMA) is a form of spread spectrum technology. The Direct Sequence Code Division Multiple Access systems will henceforth be referred as the DS-CDMA systems. In a typical DS-CDMA system, each user is allocated a different spreading code. It is the difference between the spreading codes that allows the receiver to discriminate between different simultaneous transmissions. IS-95 standard for digital cellular telephony and third generation (3G) cellular systems are based on DS-CDMA. In contrast to the typical DS-CDMA systems, there exist systems in which all users use the same spreading code. This means that every symbol sent by every user modulates the same spreading sequence, and the spreading code is a short spreading code. This type of a system is used in Wireless Local Area Networks, henceforth referred as WLAN, based on the IEEE 802.11b standard. This standard allows variable rates of data transfer, with allowed values being 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps. Signals sent at the rate of 1 Mbps and 2 Mbps are sent using direct sequence signaling. Spread Aloha system proposed by Abramson is another system in which multiple users access a channel randomly and modulate same short spreading code. The short spreading code in this case enhances the capture effect, in which a user may be able to capture the channel despite a collision. This is achieved by virtue of factors such as having higher power, or arriving at a smaller delay, so that the receiver can lock on to it and reject interference arising from collisions.
Like all outdoor wireless communication systems, Spread Spectrum systems face the problem of multipath phenomenon. When a radio frequency signal is transmitted, it comes across a lot of obstacles like buildings, beams etc. Existence of a line-of-sight path or no obstacle path is very rare. The signal reflects or bounces from the surface of the obstacle. Due to this, the receiver may receive multiple delayed versions of the same signal. These delayed versions also have different gains associated with them depending upon the different paths followed by them. This is called as the multipath phenomenon. The different reflections of the same signal received are termed as multipath components. The multipath phenomenon adds to the complexity of signal reception in the Spread Spectrum systems. In case of DS-CDMA systems, different multipath components can be associated with different users on the basis of spreading codes associated with them. However, in case all users employ same spreading code, differentiating different users' signals becomes complicated.
Considerable amount of research has been undertaken in the field of spread spectrum systems employing same spreading code for all users. Quite a few patents have been granted and research papers have been published in this regard. One such patent is recited below.
U.S. Pat. No. 5,537,397, titled “Spread ALOHA CDMA Data Communication” describes a system in which multiple transmitters transmit signals using identical spreading code and a single matched filter at the receivers receives all the signals. The matched filter transmits a control signal having the timing information of the received signal. This information is used to control the user transmission times at the chip level. At the receiver end, this information is used to differentiate between signals of different users.
A paper titled “Evaluation of the timing synchronization algorithms for the smart wireless LAN system” by G. T. Okamoto, published in 49th IEEE Vehicular Technology Conference, discusses timing synchronization algorithms that enable multiple (up to 11) users to use the same spreading code without a loss of the spreading gain. This paper considers an antenna array that is used to support Space Division Multiple Access (SDMA) in an 802.11b compatible wireless LAN.
One or more of the above described prior art suffers from one or more of the following drawbacks. Firstly, additional information is required to be sent along with the data in order to differentiate between different users' signals at the receiver. Secondly, all aspects are not taken care of to maximize the input power at the receivers end. For example, multipath phenomenon is not taken care of while receiving the signal.
From the discussion put forth, it is evident that there is a need for better receiver techniques for the direct sequence spread spectrum systems in which all users employ the same spreading code. There is need for techniques that can distinguish between the different users even though all users modulate the same spreading code without transmitting any additional information. Also, there is a need for techniques that identify multipath components of different users and reproduce the original transmitted signals.